The goal of the proposed research is to study the etiology of drug use using a sample of 211 pairs of same-sex twins reared apart from early childhood and a matched control sample of twins reared together. The twins have been identified in the Finnish twin register, one of the largest such registers in the world. The design will permit powerful analyses of hereditary influence as well as environmental influences of various kinds on the use of common drugs such as tobacco, alcohol, tranquilizers and sleeping pills. A major set of analyses will focus on the etiology of polydrug use through analyses of "genetic correlations" and "environmental correlations." Specifically, the design will permit analyses of: (1) the effect of age of separation of twins in smoking and non-smoking families in order to assess the influence of early "passive smoke" on later tobacco use; (2) the effect of age of separation in order to assess the influence of early rearing similarity on later similarity in drug use; (3) the interaction between the etiology of drug use and age; (4) the interaction between the etiology of drug use and sex; (5) the extent to which etiological factors involved in extreme drug use are similar to the etiology of less extreme drug use; and (6) genotype-environment interaction.